(1) Field of the Invention:
The present invention relates to a water-in-oil emulsion explosive composition, and more particularly relates to a water-in-oil emulsion explosive composition containing a gas-retaining agent, which has a specifically limited particle size, and having a low detonation velocity and an excellent sympathetic detonation performance without noticeable lowering of the strength.
(2) Description of the Prior Art:
Various investigations have been recently made with respect to water-in-oil emulsion explosive (hereinafter, abbreviated as W/O explosive). For Example, as disclosed in U.S. Pat. No. 3,161,551 and No. 3,447,978, the W/O explosive has an emulsified structure consisting of a continuous phase which consists of carbonaceous fuel component, and a disperse phase, which consists of an aqueous solution of inorganic oxidizer salt, such as ammonium nitrate or the like, and is entirely different in the structure from hitherto been known oil-in-water slurry explosive (hereinafter, abbreviated as O/W explosive).
That is, O/W explosive has an oil-in-water structure, wherein an aqueous solution of oxidizer salt, a sensitizer and the like are dispersed in the form of a gel together with a gelatinizer as described, for example, in Makoto Kimura, "Slurry Explosive, Performance and Use Method", Sankaido (1975). On the contrary, W/O emulsive has a water-in-oil microfine structure, wherein microfine droplets consisting of an aqueous solution of inorganic oxidizer salt and having a particle size of 10 .mu.m-0.1 .mu.m are covered with a very thin film of oil consisting of a carbonaceous fuel component and a surfactant as described, for example, in Kogyo Kayaku Kyokei-Shi, 43 (No. 5), 285-294 (1982).
W/O emulsion is remarkably different from O/W emulsion in the performance and composition due to the above described difference in the structure. That is, O/W explosive requires to contain a sensitizer, such as aluminum (U.S. Pat. No. 3,121,036), monomethylamine nitrate (U.S. Pat. No. 3,431,155 and No. 3,471,346) or the like, and is relatively low in the detonation velocity. On the contrary, W/O explosive is good in the contact efficiency of the carbonaceous fuel component with the inorganic oxidizer salt, and hence the W/O explosive has excellent properties. For examples, the W/O explosive is high in the detonation velocity, has cap-sensitivity in itself without containing sensitizer, is good in after-detonation fume, and can be changed widely in its consistency.
However, in order to maintain cap-sensitivity, propagation property of detonation, and sympathetic detonation property in a W/O explosive, the explosive must contain bubbles. As the gas-retaining agent, hollow microspheres having a small particle size are generally used. For example, U.S. Pat. No. 4,110,134 discloses the use of glass microballoons and Saran resin microballoons which form rigid independent bubbles and have a particle size of 10-175 .mu.m; Japanese Patent Laid-Open Application No. 84,395/81 discloses the use of shirasu (shirasu is a kind of volcanic ash); and a U.S. patent application Ser. No. 627,781 filed July 5, 1984 discloses the use of resin microballoons. All of these prior arts use hollow microspheres having a particle size of not larger than 175 .mu.m. Alternatively, U.S. Pat. No. 4,008,108 discloses the use, in place of the use of these gas-retaining agents, of simple bubbles mechanically flown into an explosive or simple bubbles formed from a foaming agent and the like. However, the simple bubbles as such as cannot be contained in the resulting W/O explosive in an amount more than a certain amount, are difficult to be contained in the W/O explosive for a long time, and leak from the explosive with the lapse of time, and hence the explosive loses its cap sensitivity and deteriorates in a short time, and is not advantageous for practical use. Accordingly, in order to maintain the cap sensitivity and sympathetic detonability and to obtain the detonation liability in a W/O explosive, there have hitherto been predominantly used hollow microspheres formed of a relatively hard substance and having a small particle size and being capable of forming independent bubbles and retaining the bubbles for a long time.
However, the above described W/O explosive is generally higher in the detonation velocity than O/W explosive, and the production of W/O explosive having a low detonation velocity has been difficult. For example, in order to produce a W/O explosive having a low detonation velocity, a W/O explosive having a low specific gravity or an extremely low strength is produced. However, in order to lower the specific gravity, even when a large amount of the above described hollow microspheres having a small particle size are used so as to contain 40% by volume, based on the volume of the resulting W/O explosive, of bubbles in the explosive, a W/O explosive having a detonation velocity under unconfined state of not higher than 3,000 m/sec cannot be obtained. Moreover, when such large amount of gas-retaining agent is used, the sympathetic detonability and detonation reliability of the resulting W/O explosive are greatly decreased, and the explosive cannot be practically used. Alternatively, in order to decrease the strength, a large amount of inactive substance of flame coolant, such as sodium chloride, water or the like, is used, the obtained results are the same as the above described results in the use of a large amount of microspheres, that is, due to the presence of a large amount of inactive substance, a W/O explosive having a detonation velocity of not higher than 3,000 m/sec under unconfined state cannot be obtained, and further the resulting W/O explosive deteriorates rapidly with the lapse of time, is poor in the sympathetic detonability, has a broken fine structure and has no cap-sensitivity.
When ordinary explosive is used in a place, wherein combustible gases, such as methane and the like, or combustible dusts, such as coal dust and the like, are present, there is a risk of gas explosion or dust explosion. Such operation site, for example, coal mine or like is in duty bound to use an explosive having a safety higher than a given safety standard. In order to produce an explosive having a high safety against methane, coal dust and the like, it is indispensable to decrease the strength of explosive and further to decrease the detonation velocity. Particularly, in a W/O explosive having a relatively high detonation velocity, in order to obtain the same safety as that of W/O explosive, the strength of the W/O explosive must be extremely lowered (for example, Japanese Patent Laid-Open Application No. 155,091/81). However, a W/O explosive having an extremely low strength is poor in the detonation liability, sympathetic detonability and storage stability, and cannot be practically used. Moreover, the use of an explosive having a low strength is poor in the mining effect and increases the number of blasting times, resulting in an increased danger.
The inventors have variously studied in order to produce a W/O explosive having a low detonation velocity, a high safety and an excellent sympathetic detonability without decreasing extremely its strength, and surprisingly found out that the use of a specifically limited gas-retaining agent can produce effectively a W/O explosive composition having a low detonation velocity and an excellent sympathetic detonability, and have reached the present invention.